This invention refers to the production of soft-core and coreless rolls of films or continuous band of thin sheet material such as stretchable and heat shrinkable plastic films, or bands in paper or different materials, which are continuously fed, embossed and wound or rolled up onto a spindle to form a roll in a controlled mode.
More precisely, the invention relates to a method and an apparatus for producing soft-core and coreless rolls of a film material namely devoid of a support tube, or provided with a support soft tubular core for winding up the film. For the purpose of the present invention, “soft-core” means a core of soft or flexible material, such as paper and the like, for winding up the film, compared to rigid cardboard cores.
The invention is also directed to a coreless or soft-core roll of film material obtained according to the claimed method.
Although the formation of coreless or soft-core rolls for example of plastic film, currently has a specific use in the packaging or for winding palletised loads with a pre-stretched film, this invention is nevertheless applicable to the formation of coreless rolls or provided with a soft tubular core, by any type of extensible or heat-shrinkable plastic films, paper or different sheet material depending upon the circumstance and use.
It is well known that heat-shrinkable or extensible plastic films wound into roll, are normally used for rolling up and packaging palletised loads or goods.
The use of extensible and heat-shrinkable plastic films is widespread in the packaging field, in that it offers the possibility of adequately consolidating any type of palletised load or packaged goods, by simply winding around and making the film adhere to the load or good with a certain tension.
Furthermore, the use of pre-stretched plastic films proves to be advantageous in that pre-stretching gives the plastic material greater resistance, and in that a pre-stretched film can be wound around a load, especially delicate loads or goods, without causing excessive stress.
However, in winding up rolls of film, and in their subsequent use, several problems arise which have been variously tackled and variously solved in the past.
In fact, with usual smooth films, it is difficult to wind coreless rolls or to wind up the film on a soft tubular core, which has a stable shape, due to their tendency to implode.
Moreover, pre-stretched films or elastically yielding films retain an elastic memory which over time makes them to shrink in such a way that the outer turns generate a pressure on the inner turns, deforming and making them adhere to one another, thereby preventing or making it difficult to properly unwind the stretched film from a roll.
This problem has been partially solved by suitably embossing the film, for example by forming a plurality of small pockets, by slightly deforming the film against an appropriate toothed embossing drum, trapping the air and preventing a close contact between the turns of the roll.
An apparatus for embossing extensible film wound up into a roll is described for example in EP-A-0 728 102.
According to this document, the plastic film is made to move, at a constant speed, between a set of drawing rollers connected to a control motor; a toothed drum embosses the film, before it being wound onto a roll, maintaining the plastic film under a stretched condition so as to make it frictionally adhere to the toothed surface of the embossing drum; the teeth of the embossing drum cause a partial deformation of the plastic film and the consequent formation of small pockets in which air remains trapped during a subsequent winding step of the roll.
This solution has however a number of drawbacks, such as for example the difficulty in obtaining evenly wound rolls having a constant diameter; in fact the deformation of the film, necessary to form the small embossed pockets for entrapping the air, tends to generate irregularly distributed internal stresses, with the consequent formation of creases in the film during the winding up of the roll. Moreover, since the air remains entrapped in the individual pockets, without any possibility of venting, the rolls of film have a final diameter, which is still considered to be excessive, in relation to the quantity of wound up film.
Lastly, the winding up of coreless as soft-core rolls by the methods and apparatuses currently in use, leads to the formation of rolls having very large diameters, with consequent higher storage and transport costs due to the larger volume incurred by the same rolls.
U.S. Pat. No. 5,003,752 describes a winding up method and an apparatus for rolling up a pre-stretched film around a load, which make use of stretching rolls each having alternate and intermeshing peripheral ridges, arranged such that the ridges on each roll mesh with the grooves on the other roll; the plastic film introduced between the stretching rolls, is continuously folded in the longitudinal direction and forcibly drawn, stretched longitudinally and in the crosswise direction; the stretched film emerging from the stretching rolls to be wound around the load, is again in a smooth or flat form.